In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Recently cellular operators have begun offering mobile broadband based on the Long Term Evolution (LTE) wireless system. Fuelled by new devices designed for data applications, the end user performance requirements are steadily increasing. Consequently, techniques that enable cellular operators to utilize their spectrum resources more efficiently are of increasing importance. Table 1 shows various downlink transmission modes for Long Term Evolution (LTE), including various MIMO modes.
TABLE 1LTE Transmission ModesTransmission ModeDownlink Transmission SchemeMode 1Single Antenna Port (SISO or SIMO)Mode 2Transmit DiversityMode 3Open-Loop Spatial MultiplexingMode 4Closed-Loop Spatial MultiplexingMode 5Multi-User MIMOMode 6Closed-Loop Rank-1 SpatialMultiplexingMode 7Single Antenna Port BeamformingMode 8Dual-Layer Beamforming
MIMO (multiple input and multiple output) is one of the key technologies that provides substantial improvements in spectral efficiency. MIMO involves the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. To do so, MIMO spreads the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency (more bits per second per hertz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading).
Currently, there are two widely used MIMO techniques: OL-MIMO (open loop MIMO) and CL-MIMO (closed loop MIMO). As summarized in a white paper entitled “Maximizing LTE Performance Through MIMO Optimization” (http://rfsolutions.pctel.com/artifacts/MIMOWhitePaperRevB-FINAL.pdf), open loop and closed loop modes differ in the level of detail and frequency with which channel conditions are reported by the UE. The eNodeB relies on detailed and timely information from the UE in order to apply the best antenna and data-processing techniques for the existing channel conditions. Depending on the UE's data-processing speed as well as the quality of its connection to the eNodeB in both uplink and downlink, LTE will operate in either closed loop or open loop mode.
The eNodeB communicates with a UE in open loop when the UE is moving too fast to provide a detailed report on channel conditions in time for the eNodeB to select a precoding matrix. Other factors, such as UE processing speed or uplink data capacity (which may also be affected by UE specifications), may result in open loop operations even when the UE is moving relatively slowly. The UE's capabilities are therefore crucial for achieving the best results from particular multipath conditions. In open loop operations, the eNodeB receives minimal information from the UE: (1) a Rank Indicator (RI) which indicates the number of layers that can be supported under the current channel conditions and modulation scheme; and (2) a Channel Quality Indicator (CQI) which is a summary of the channel conditions under the current transmission mode, and which roughly corresponds to the signal to noise ratio (SNR). The eNodeB then uses the CQI to select the correct modulation and coding scheme for the channel conditions. Combined with this modulation and coding scheme, CQI can also be converted into an expected throughput. The eNodeB adjusts its transmission mode and the amount of resources devoted to the UE based on whether the CQI and RI reported by the UE matches the expected values, and whether the signal is being received at an acceptable error rate.
In closed loop operations, the UE analyzes the channel conditions of each transmission (Tx), including the multipath conditions. In the closed loop MIMO, the receiver reports channel status to the transmitter via a special feedback channel, making it possible to respond to changing circumstances. In particular, in closed loop MIMO the UE provides a RI; one or two CQI reports depending on the RI value; and a Precoding Matrix Indicator (PMI).
Precoding is well known in the art and, in general, is applied to the data carried on the Physical Downlink Shared Channel (PDSCH) in order to increase the received Signal to Interference plus Noise power Ratio (SINR). This is done by setting different transmit antenna weights for each transmission layer (stream) using channel information fed back from the UE. The ideal transmit antenna weights for precoding are generated from eigenvector(s) of the covariance matrix of the channel matrix, H, given by HHH, where H denotes the Hermitian transpose. LTE Rel. 8 uses codebook-based precoding, in which the best precoding weights among a set of predetermined precoding matrix candidates (a codebook) is selected to maximize the total throughput on all layers after precoding, and the index of this matrix (the Precoding Matrix Indicator (PMI)) is fed back to the base station (eNode B).
FIG. 9A illustrates some basic aspects of conventional closed loop MIMO operation. Act A-1 of FIG. 9A shows the wireless terminal sending one or more report(s) of numerous parameters (including RI, the Precoding Matrix Indicator (PMI), HARQ ACK/NAK, and CQIs for two codewords) to the node. As act A-2 the base station node evaluates the wireless terminal's reports. Act A-3 comprises the base station node making its decision regarding the appropriate transmission mode. Then, as act A-4, the base station node communicates its decision of the transmission mode to the wireless terminal using higher layer signaling. Upon receipt of the transmission mode decided by the base station node as act A-5 the wireless terminal implements and acknowledges the transmission mode decided by the base station node. Until the time of such acknowledgement, the base station operates with some ambiguity. Upon receipt of the confirmation from the wireless terminal, as act A-6 the base station node then begins to schedule transmissions to the wireless terminal using the new transmission mode, e.g., the transmission mode decided at act A-3. Aspects of conventional open loop MIMO employ acts similar to those shown in FIG. 9A, it being understood, however, that in open loop MIMO for act A-1 the wireless terminal does not transmit the PMI and for each parameter transmits a value averaged over two codewords rather than value for each of two codewords.
Simulation results appear to indicate that CL-MIMO has better performance than OL-MIMO when the mobile speed is low. However, when the mobile speed is high, the advantage of CL-MIMO vanishes.
Given the fact that in CL-MIMO, the receiver reports channel status on an uplink control channel to the transmitter, a problem with conventional CL-MIMO in LTE is that the uplink control channel overhead increases in direct proportion to the number of users. Specifically, LTE CL-MIMO requires two channel quality (CQI) reports plus one PMI report per user.
For a given UE the LTE system can switch between the CL-MIMO and OL-MIMO modes. But the transition or switching may be slow. The transition between modes requires Radio Resource Control (RRC) signaling which (typically) takes 100s of milliseconds. This reduces the ability of LTE to respond to rapidly changing radio frequency (RF) channel conditions, which in turn impacts system capacity.